I found an odd bug a while ago when I created ring textures for some of the planets I made: the gaps in the rings were not displaying properly when lit from behind. Even when tried to make doubly sure that that part of the texture was transparent, it would not appear properly. I had originally made all the textures using an older version of Adobe Photoshop Elements.I recently found that if I opened the texture file and saved a copy in Paint 3D, even without making any edits, that gaps would render properly. Why would that make a difference?

I don't think this is an SE bug, but rather the image format. Technically, png file format does not support transparencies. It would appear that Adobe sticks to the letter of the law with the format, while Paint 3D works around it. (Big file size difference between the two also!)